Coherent laser radar apparatus

ABSTRACT

A coherent laser radar device is described which comprises a laser source ( 11 ), and two or more transceivers ( 84 ) are optically connected to the laser source by optical fiber cables ( 42, 48, 82 ) that are routed through an optical routing means ( 80 ). A detection means ( 27 ) is also provided, and radiation received by the two or more receivers ( 84 ) can also be optically coupled to the detection means ( 27 ) by optical fiber cables ( 82, 86, 90 ).

This invention relates to coherent laser radar apparatus, and moreparticularly to low cost coherent laser radar apparatus for thedetermination of wind speed at a plurality of positions.

Coherent laser radar (CLR) systems have been developed over many yearsfor wind speed measurements as well as for hard target measurements,such as ranging. The basic principle of CLR is to direct a laser beam toan object and to detect the returned signal. If the laser beam isfrequency modulated, comparison of the phase of the emitted and detectedsignal provides range information. Also, any Doppler shift in thefrequency of the detected signal will give information on the relativespeed of the object. The part of a CLR system which transmits andreceives radiation is termed the transceiver. The transceiver may havecommon or separate transmit and receive optics; such transceivers aretermed mono-static and bi-static transceivers respectively.

Wind speed measurement techniques rely on the assumption that airborneparticles will be moving at the same speed as the air in which they aresuspended. Thus, wind speed may be determined by measuring the Dopplershift in laser light reflected from the particles passing through aparticular volume of air. The particular volume is commonly termed theprobe volume. Wind speed measurements of this type are presently used inaircraft to determine their relative air speed.

Although CLR devices can readily provide accurate wind speedmeasurements, the systems are typically bulky in size and are alsocostly to build, maintain and operate. For example, the 10.5 μmwavelength carbon dioxide laser generally requires water cooling and theassociated 10.5 μm infra-red detector often needs to be gas cooled.

An alternative CLR system is described in Karlsson et al, AppliedOptics, Vol. 39, No. 21, 20 Jul. 2000. The system described in Karlssonet al is an all fibre multifunction continuous wave CLR device whichoperates at a wavelength of 1.55 μm. Although such a device is lessbulky than traditional systems, a laser diode and erbium doped opticalamplifier are required. These components, although not requiring gas orliquid cooling, are relatively bulky and expensive.

A significant drawback of acquiring wind measurements using known CLRsystems is that the measurement of wind speed can generally only beobtained for a single probe volume using a single CLR device. Typically,the measurement of wind speed in multiple probe volumes has required theuse of multiple CLR devices. Although the simultaneous measurement ofmultiple probe volumes would prove advantageous in many applications(e.g. avionics), the provision of multiple CLR devices has beenconsidered impractical due to cost, weight and size considerations.

U.S. Pat. No. 5,048,951 describes a device in which a single laser beamis sequentially directed along, and the return beam is detected from,two or more different directions. In particular, U.S. Pat. No. 5,048,951describes an anemometer in which the, wind speed is measured in threeorthogonal directions thereby providing the three components of thespeed of an aircraft with respect to air.

This system of U.S. Pat. No. 5,048,951 overcomes to some extent thecost, size and weight problems associated with making wind speedmeasurement in multiple probe volumes. However, the total number oftransmit/receive directions and the angular separation between each ofthese directions is limited by the compactness of the components of theoptical system. An additional disadvantage of the system described inU.S. Pat. No. 5,048,951 is the requirement to maintain the variousoptical components in precise alignment. Also, the provision of a pairof rotating reflective disks to direct the radiation in differentdirections is prone to failure.

It is the object of this invention to mitigate at least some of thedisadvantages described above.

According to a first aspect of the invention, a coherent laser radardevice comprises a laser source, and two or more transceivers and ischaracterised in that the two or more transceivers are opticallyconnected to the laser source by optical fibre cables, and the opticalfibre cables connecting the laser source and the two or moretransceivers are routed through a first optical routing means.

The provision of a single laser source coupled to two or moretransceivers is (transceivers being apparatus for both transmitting andreceiving radiation) by optical fibre cables has several advantages overprior art devices.

The optical fibre linkage allows the two or more transceivers to belocated in positions that are remote to the laser source. For example,if the CLR device is deployed to measure wind speed each transceiver maybe located at various positions along an aircraft wing or on two or morewind turbines.

The use of a single laser source coupled to the two or more transceiverheads through the first optical routing means also has advantages interms of the cost, size and weight of the CLR device. For example, theprovision of multiple prior art CLR devices on an aircraft to measurewind speed at multiple locations would not be practical due to the costand weight of each device. However, a device according to the presentinvention could be used to measure the relative wind speed of multipleprobe volumes.

Similarly, the present invention overcomes disadvantages of the devicesof the type described in U.S. Pat. No. 5,048,951. For example, thetransceivers of a device according to the present invention may belocated in any position relative to the laser source and may be directedto probe volumes or objects in any direction relative to the lasersource. This is an advantage over the type of device described in U.S.Pat. No. 5,048,951 in which the direction of the CLR measurements arelimited by the physical configuration of the beam splitting apparatus.

Advantageously, the first optical routing means is a multi-way opticalsplitter. A multi-way optical splitter allows the optical power of thelaser source to be split between the transceivers. Simultaneousmeasurement of range, or wind speed etc from the various transceiverheads is thus possible. The laser power available to each of thetransceivers, and hence the strength of any received return signal, isthe only limit on the number of transceivers that can be used in such aconfiguration.

Conveniently, the first optical routing means is an optical switch; theoptical switch directing laser power to one transceiver at a time. Thesequential direction of power to each transceiver in turn allowsquasi-simultaneous measurements to be acquired. This type ofquasi-simultaneous measurement is particularly suited to operation whenthe observed phenomena (e.g. wind speed, range etc) shows an evolutionover time that is slower than the switching cycle time scale.

In a further embodiment the optical switch is a wavelength divisionmultiplexer (WDM), the WDM being arranged so as to direct laserradiation of different wavelength to different transceivers.

Tuning the output wavelength of laser radiation from the radiationsource causes the WDM to direct the radiation to a particulartransceiver. The WDM, which may be a dense WDM (DWDM), has no moving ormechanical parts and is thus physically robust.

Advantageously, the optical switch is an optical fibre switch that iscontrolled to direct laser radiation to a particular transceiver head.Electronically controlled optical fibre switches are commerciallyavailable, and allow radiation to be routed to a particular transceiveras required.

In a further embodiment a single detector is provided, and the radiationreceived by the two or more transceivers is optically coupled to thedetector by optical fibre cables via a second optical routing means. Inother words, the second optical routing means allows the return signalfrom one of the two or more transceivers to be directed to the detector.A person skilled in the art would recognise the type of detectorappropriate for detecting the returned laser signal.

Conveniently, the second optical routing means is an optical switch suchas a WDM, and the frequency of the laser source is alterable such thatradiation received by a particular transceiver head is directed to thedetector.

The use of a WDM, or more preferably a DWDM, as the second opticalrouting means would be particularly suited to configurations where laserradiation is provided to, and transmitted by, all transceivers;wavelength tuning of the laser output being used to select thetransceiver for which received radiation is to be detected.

Advantageously, the optical switch is a fibre switch that is controlledsuch that radiation received by a particular transceiver head isdirected to the detector.

In a further embodiment the radiation directed to the detector iscoherently mixed with radiation extracted from the laser source prior todetection by the detector. A person skilled in the art would appreciatethat subsequent electrical mixing of the signal produced by thedetector, so-called “beating down”, may also be advantageous when thereis a significant frequency difference between the return signal and thelocal oscillator signal.

In another embodiment, radiation received by the two or moretransceivers is directed to two or more detectors. Two or more detectorsshould also be taken to include a detector having two or more detectionelements.

Advantageously, the radiation directed to each of the two or moredetectors is coherently mixed with radiation extracted from the lasersource prior to detection by the detector.

Conveniently, the detectors are coupled to each transceiver by anoptical fibre cable. This allows the detector to be located remotely tothe transceiver, and a optical fibre link is provided to coupleradiation between them.

Advantageously, the transceivers are monostatic; i.e. the transceiverhas common transmit and receive optics. The transceivers may also bebistatic; i.e. the transceiver has separate transmit and receive optics.

Advantageously, the device has more than three, more than five or morethan ten transceivers.

In a further embodiment, the transceivers are located remotely to thelaser source. The transceivers may be located tens of meters, hundredsof meters or even kilometers from the laser source. The maximum lengthof optical fibre cable is limited only by the acceptable level ofoptical loss.

Conveniently, the laser source is a semiconductor laser or asemiconductor laser and an erbium doped amplifier arranged in a masteroscillator power amplifier configuration. These laser sources meet therequirements of having a sufficiently long coherence length, and a lowrelative intensity noise (RIN).

Advantageously, the laser source outputs radiation with a wavelengthbetween 1.4 μm and 1.6 μm.

In a further embodiment, the detected signals are processed to obtainspeed information, such as the speed of particles in air to provide windspeed measurements. The laser source may also be frequency modulated sothat the detected signals can be processed to provide range information.Vibrometric information may also be provided. A person skilled in theart would recognise the various processing techniques that areappropriate for the extraction of such information from the detectedsignals.

Conveniently, the apparatus used to process the detected signals islocated remotely to the transceivers; for example the apparatus used toprocess the detected signals may be located substantially adjacent tothe laser source.

According to a second aspect of the invention, a wind speed measurementsystem for use on an aircraft incorporates a device according to thefirst aspect of the invention, wherein the transceivers are locatedalong the wings of the aircraft and are directed so as to measure thewind speed of probe volumes in front of the aircraft, and the measuredwind speed information is used to adjust the configuration of theaircraft wings.

Aircraft wind gust sensors using a device according to the first aspectof the present invention can provide measurement of air flow at multiplepoints in front of the aircraft wings so that the wing shape (e.g. flapsettings) can be automatically adjusted to optimise efficiency and ridequality. Also, measurement of the wind speed of a point from threedifferent directions would provide three dimensional wind fieldinformation that could prove valuable when landing an aircraft on aship. The present invention provides a practical and weight/costeffective way of performing such measurements.

According to a third aspect of this invention, a wind speed measurementsystem for ship landing applications incorporates a wind speedmeasurement device according to the first aspect of the invention thathas at least three transceivers, wherein the transceivers are spatiallyseparated and are adapted to measure the wind speed of a single probevolume; the measurement of the wind speed of a probe volume from threedifferent directions permitting the three dimensional wind field vectorfor the probe volume to be ascertained.

In other words, measuring wind speed of the same probe volume from atleast three different directions permits the wind speed of the probevolume in at least three different directions to ascertained. Thisallows the wind velocity for the probe volume to be calculated, whichcould prove advantageous for aircraft (including helicopters) landing onships. The wind speed measurement system could be deployed on theaircraft, or on the ship itself.

According to a fourth aspect of the invention, a wind velocitymeasurement system for measuring wind strength in front of two or morewind turbines incorporates a wind speed measurement device according tothe first aspect of the invention wherein the two or more transceiverheads are located on each wind turbine and the laser source is locatedin the base of one of the wind turbines.

A device according to the first aspect of the present invention can thusbe used on wind farms to measure the wind strength in front of each windturbine so that the various turbine parameters can be adjusted inresponse to the incident wind strength. The present invention provides acost effective way of performing such measurements.

The invention will now be described, by way of example only, withreference to the following drawings in which;

FIG. 1 shows a prior art Doppler CLR device,

FIG. 2 shows a device according to the present invention,

FIG. 3 shows an alternative device according to the present invention,

FIG. 4 shows three additional devices according to the presentinvention; and

FIG. 5 shows uses of devices according to the present invention.

Referring to FIG. 1, prior art mono-static CLR Doppler device of thetype described in Karlsson et al, Applied Optics, Vol. 39, No. 21, 20Jul. 2000, the contents of which are incorporated herein by referencethereto, is shown.

The CLR device comprises a distributed feedback (DFB) diode laser 2 thatemits radiation at a wavelength of 1.55 μm, and a laser power supply 4.The output intensity of the DFB diode laser 2 can be frequency modulatedby applying an electrical modulation signal from a waveform generator 8to the laser; such frequency modulation is required for rangemeasurements. The laser beam generated by the diode laser 2 is coupledinto an optical fibre cable 6. The laser beam exits the fibre opticcable 6, and is amplified by an erbium doped amplifier 10. Thearrangement of the diode laser 2, the laser power supply 4, the opticalfibre cable 6 and the erbium doped amplifier 10 in a master oscillatorpower arrangement (MOPA) is hereinafter collectively termed a lasersource 11.

The amplified laser beam is coupled, via a fibre optic cable 12, to apolarisation independent fibre-optic circulator 14. The fibre-opticcirculator 14 comprises a plurality of discrete optical components (notshown). These optical components are arranged such that the amplifiedlaser beam incident from optical fibre cable 12 is output to opticalfibre cable 16. Any radiation incident on the fibre-optic circulator 14from optical fibre 16 is also transmitted to the optical fibre 18.

A telescopic head 20 contains a doublet lens (not shown) which focusesthe laser beam emerging from the end of the fibre optic cable 16 to aparticular point. Movement of the end of the fibre optic cable relativeto the doublet lens allows the transmitted laser beam to be focussed atvarious distances from the telescopic head. The transceiver head ismono-static; the doublet lens therefore also acts to focus receivedradiation such that it is coupled in to the optical fibre 16.

The local oscillator signal required for coherent detection is generatedby Fresnel reflection from the end of the fibre optic cable 16 that iscoupled to the telescopic head 20. All the other fibre ends used in thedevice (i.e. both ends of fibres 6, 12 and 18 and the end of fibre 16which couples to the fibre-optic circulator 14) are angled so as tominimise reflections.

The telescopic head 20 and the fibre-optic circulator 14 thus form atransceiver 21. The laser beam input to the transceiver 21 via opticalfibre 12 is transmitted to a remote point, and the radiation reflectedfrom that point and received by the transceiver is output via opticalfibre 18.

A detector unit 22 receives radiation from the optical fibre 18. Theradiation incident on the detector unit 22 is a coherent mix of thesignal received by the telescopic head 20 and the local oscillatorsignal derived from Fresnel reflection from the end of the fibre opticcable 16. The detector unit 22 comprises a InGaAs photodiode and atransimpedance amplifier (not shown).

The electrical signal generated by the detector unit 22 is passedthrough an anti-alias filter 24 before being converted to a digitalelectrical signal by an analogue-to-digital converter 26. Collectivelythe detector unit 22, anti-alias filter 24 and analogue-to-digitalconverter 26 are herein termed detection means 27. A personal computer28 is used to analyse the digital electrical signal provided by thedetection means 27, and is also used to control the modulation signalthat is applied to the diode laser 2 by the waveform generator 8. Aperson skilled in the art would recognise that a surface acoustic wave(SAW) spectrum analyser could also be used to perform the necessaryanalysis of the electrical signal.

As described in more detail in Karlsson et al, control of the outputlaser beam, and analysis of the returned signal, can yield range orspeed measurements. This includes measurements of wind speed in aparticular probe volume. However, this prior art device is limited tomeasuring the range of a single point or the wind speed in one probevolume.

Referring to FIG. 2, a CLR device of the present invention is shown.Elements of the device described with reference to FIG. 2 that arecommon to the description of FIG. 1 are given like reference numerals.

A laser source 11 emits a laser beam that is coupled into an opticalfibre cable 42. A beam splitter 44 is provided and directs a smallfraction of the laser power as a local oscillator signal to opticalfibre cable 46, and the remaining optical power is directed in tooptical fibre cable 48. A person skilled in the art would recognise thatthe optical power of the local oscillator signal would advantageously beadjusted to give optimised shot noise domination in the detector.

A three way beam splitter 50 equally divides the laser power incidentfrom optical fibre cable 48 between the optical fibre cables 52 a, 52 band 52 c, which in turn are coupled to transceivers 54 a, 54 b and 54 c.Each of the transceivers 54 transmit the laser radiation, and alsooutput any received radiation (i.e. radiation reflected back to it froman object) to their respective optical fibre cables 56.

Optical mixers 58 coherently mix the received radiation of each of theoptical fibre cables 56 with the local oscillator signal provided by thebeam splitter 44. The resultant coherently mixed signals are outputalong optical fibre cables 59 to each of the respective detection means27. A personal computer 60 processes the data provided by each of thedetection means 27 generating range or speed data as required. The CLRdevice thus provides three simultaneous measurements of range and/orspeed for the three transceivers; however this is at the cost of eachtransceiver requiring its own detection means 27.

The device of the type shown in FIG. 2 can be considered to have acentral unit 64 and a plurality of transceivers 54 linked by the opticalfibre cables 52 and 56. The length of the optical fibre cables 52 and 56may be many ten's of meters, or even several kilometers, as required;the only limitation on the length of the cable is the optical loss whichit introduces. The use of optical fibre cable allows the positioning ofthe transceivers at substantial distances from each other, and away fromthe central unit 64.

A person skilled in the art would also appreciate that the transmissionof high power coherent laser light through long lengths of single-modefibre (e.g the optical fibre cables 52) can lead to an increase in thenoise levels arising from non-linear optics (NLO) mechanisms such asstimulated Brillouin scattering (SBS). NLO effects depend on fibrelength, and a particular fibre will have an optical power thresholdabove which NLO noise starts to significantly effect device performance.More detail on NLO effects can be found elsewhere; for example see DCotter, Electronics Letters 18 (12) 495–496 (1982).

It has been found that a typical optical power threshold for a standardsingle mode fibre is around 1 W with a 125 m length of optical fibre,and approximately 80 mW with a 4 km length of optical fibre cable. Aperson skilled in the art would however appreciate the various ways inwhich the NLO effects could be mitigated. For example, the SBS mechanismcould be suppressed by reducing feedback in the transmit fibre usingoptical isolators (not shown) located at regular intervals along thetransmit fibre. The required spacing of the optical isolators woulddepend upon the transmitted optical power.

Alternatively, NLO effects could be reduced by locating an opticalamplifier, such as an Erbium doped fibre amplifier (EDFA), close to eachtransceiver or cluster of transceivers thus ensuring only low-powertransmission along the main transmit fibre. The threshold for the SBSprocess may also be increased by using single-mode fibres that have anincreased mode area, such as photonic crystal fibre (PCF). Furthermethods (e.g. see T Imai et al, Elec. & Comm. in Japan 78 (11) 22–31(1995)) include using fibres that enlarge the SBS gain bandwidth byapplying a varying strain distribution along the length of the fibre, orby shifting the frequency of the SBS spectrum by varying the core dopantconcentration along the length of the fibre.

Furthermore, it should be recognised that the function of coherentlymixing the local oscillator optical beam with the received optical beamsneed not be performed in the central unit 64. For example, the coherentoptical mixing could be performed using fibre end reflections in thetransceiver itself as described above with reference to FIG. 1.

It should also be noted that although only optical mixing is performedin the device described with reference to FIG. 2, subsequent mixingcould also be performed in the electronic domain. A person skilled inthe art would appreciate that subsequent electrical mixing is generallyrequired when there is a significant frequency difference between thereturn signal and the local oscillator signal. This large frequencydifference may occur when high speeds are measured. Alternatively, alarge frequency shift between the local oscillator and return signal maybe introduced by inclusion of an acousto-optic modulator (not shown) sothat the Doppler shift measurements can distinguish relative motiontowards and away from the transceiver.

The embodiment described with reference to FIG. 2, is particularlysuited to situations where plenty of laser output power is available and“sharing” of the laser power is appropriate. Although a 1×3 split isshown, the only limit on the degree of splitting (and hence the numberof transceivers that can be used in the device) is the optical powerneeded to make the required measurements.

A person skilled in the art would also recognise that optical isolators(not shown) could be included in any portion of the transmit and/orreceive optical fibre cables as required. The inclusion of opticalisolators would eliminate noise brought about by interference of thelocal oscillator signal with internal reflections in addition toreducing NLO effects as described above.

Referring to FIG. 3, a second example CLR device of the presentinvention is shown. Elements which are like those shown with referenceto FIGS. 1 and 2 above have been assigned like reference numerals.

A laser source 11 emits a laser beam that is coupled into an opticalfibre cable 42. A beam splitter 44 is provided and directs a fraction ofthe laser power as a local oscillator signal to optical fibre cable 46,and the remaining optical power is coupled to the optical fibre cable48.

An optical switch 80 receives radiation from optical fibre cable 48, anddirects that radiation to any one of the transceivers 84 a, 84 b and 84c via the respective optical fibre cables 82 a, 82 b or 82 c. Eachtransceiver 84 also couples any radiation received (i.e. any returnedradiation) back into the relevant optical fibre cables 82, and theoptical switch 80 then directs this radiation from the selected opticalfibre cable 82 to the optical fibre cable 86.

The radiation fed into optical fibre 86 is coherently mixed, in a mixer88, with the local oscillator signal provided by the beam splitter 44.The coherently mixed radiation is routed through optical fibre cable 90to the detection means 27. Range and speed information, as required, canthen be calculated by the personal computer 92 for the particularselected transceiver.

The optical switch 80 thus has the effect of routing optical power toone transceiver (e.g. transceiver 84 b), and routing the return signalreceived by that transceiver (i.e. transceiver 84 b) to the detectionmeans 27 thereby providing range or speed information. By switching theoptical switch, the transceivers can be sequentially activated, allowingquasi-simultaneous measurements to be performed.

The optical switch 80 could be any device that is capable of routingoptical signals without any significant loss of the coherenceinformation. Such switches are commonly used in the field oftelecommunications.

For example, the switch may be a wavelength division multiplexer (WDM)or more particularly a dense wavelength division multiplexer (DWDM). ADWDM will route an incident optical beam to any one of N outputchannels; the wavelength of the incident optical beam determining whichof the output channels is selected. In this manner, the laser source ofthe CLR could be tuned to output slightly different wavelengths oflight, thereby causing the DWDM to switch the laser power to aparticular transceiver.

Alternatively, a duplex optical switch could be provided that consistsof a set of two common input fibres (i.e. for a transmit signal and areceive signal) that move as a group (i.e. synchronously) into alignmentwith a corresponding set of two output fibres.

The switching rate between transceivers is chosen to ensure the observedphenomena show time evolution at a rate slower than the switching cycletimescale. For example, a time of 10–20 ms is often sufficient for asingle reliable wind measurement to be carried out. Hence, for number oftransceivers N˜10, a full measurement cycle can be carried out over aperiod 0.1–0.2 seconds. A wind field would not normally be expected toevolve significantly over this timescale.

The simple calculation described above has ignored the switching time ofthe optical switch which, for optical fibre duplex switches, is of order10–200 ms. Therefore, the measurement timescales are slightly increasedwhen using optical fibre duplex switches. However, faster switchingtimes can be achieved using DWDM technology combined with rapid lasertuning.

A device of the type shown in FIG. 3 can be considered to have a centralunit 64 and a plurality of transceivers 84 linked by optical fibrecables 82. The length of the optical fibre cables 82 may be many ten'sof meters, or even several kilometers, as required; the only limitationon the length of the cable being the optical loss which it introduces.The use of optical fibre cable allows the positioning of thetransceivers at substantial distances from each other, and from thecentral unit 64.

Referring to FIG. 4, three alternative embodiments of the presentinvention are shown which incorporate bistatic transceivers 21 of thetype described with reference to FIG. 1.

The device of FIG. 4 a comprises a central unit 64. The central unit 64has a laser source 11, the output of which is fed through an opticalfibre cable 93 to an optical switch 94. The optical switch 94 directsradiation from the laser source 11 to any one of the three transceivers21 through the appropriate optical fibre cable 96, thereby allowing anyone of the transceivers 21 to be activated.

The optical switch 94 also functions so as to route radiation receivedby the activated transceiver 21, which will have mixed with a localoscillator signal within the transceiver 21, to a detector 99. Theelectrical signal produced by the detector 99 is analysed by dataanalysis means 100 to extract the required range, speed or vibrometricinformation. The optical switch 94 may be a duplex optical fibre switch,or a pair of DWDMs, of the type described with reference to FIGS. 2 and3 above.

The device shown in FIG. 4 b comprises a central unit 64. The centralunit 64 has a laser source 11, the output of which is fed through anoptical fibre cable 93 to an optical switch 102. The optical switch 102directs radiation from the laser source 11 to any one of the threetransceivers 21 through the appropriate optical fibre cable 96 therebyallowing any one of the transceivers 21 to be activated.

Radiation received by each of the activated transceivers 21, which willhave mixed with a local oscillator signal within the transceiver 21, isdirected to the detectors 99 via optical fibre cables 97. An electricalswitch 104 directs the electrical signal from one of the detectors 99 tothe data analysis means 100 which extracts the required range or speedinformation for the activated transceiver.

The device shown in FIG. 4 c again comprises a central unit 64. Thecentral unit 64 has a laser source 11, the output of which is fedthrough an optical fibre cable 93 to an optical switch 102. The opticalswitch 102 directs radiation from the laser source 11 to any one of thethree transceivers 21 through the appropriate optical fibre cable 96,thereby allowing any one of the transceivers 21 to be activated.

Radiation received by each of the activated transceivers 21, which willhave mixed with a local oscillator signal within the transceiver 21, isdirected to detectors 99. The detectors 99 are located in the vicinityof the transceivers 21, and the electrical signals produced are sentthrough electrical cables to the electrical switch 104. The electricalswitch 104 directs the electrical signal from one of the detectors 99 tothe data analysis means 100 which extracts the required range or speedinformation.

The devices shown in FIG. 4 allow the transceivers 21 to be locatedremotely to the central unit 64. The transceivers can be connectedoptically (as in FIGS. 4 a and 4 b) to the central unit 64 using opticalfibres 96 and 97. This is advantageous when electrical fields arepresent which could interfere with the transmission of electricalsignals. Alternatively, as shown in FIG. 4 c, the connection to thelaser source can be via a optical fibre 96 whilst the received signal iscarried electrically to the central unit 64 through the cable 103. Thisis advantageous when the received signal intensity is low, and theoptical loss and possible noise introduced by an optical fibreconnection is thus undesirable.

Referring to FIG. 5, various uses of devices according to the presentinvention are illustrated.

FIG. 5 a, shows a device of the type described with reference to FIG. 3deployed on an aircraft 120. The central unit 64 of the CLR may bepositioned anywhere within the aircraft; the transceivers 84 beinglinked to the central unit 64 via optical fibre cables 82. Thetransceivers can thus be deployed in positions where prior art CLRdevices could not; for example in the wing tips.

Each transceiver 84 is configured to allow acquisition of wind speedinformation from a probe volume 122 approximately two hundred meters infront of the aircraft. The central unit 64 determines the wind speed ofeach probe volume 122 sequentially, thereby building up a wind field mapthat allows the wing shape of the aircraft (e.g. the flaps) to beadjusted accordingly in order to increase fuel efficiency and improveride quality. Altering wing shape in response to gust measurements,which may be termed adaptive winging, could be incorporated in the nextgeneration of airliners and may improve airline safety by allowingapproaching air turbulence to be detected.

The CLR device, when deployed on an aircraft, could also be used tomeasure the 3-D wind field of a single point in space. For example, inthe configuration shown in FIG. 4 a, the transceivers 84 a, 84 d and 84h could all be focussed on to a single probe volume (e.g. probe volume124). The lateral displacement of the three transceivers 84 a, 84 d and84 h along the aircraft wing enables line of sight wind measurements tobe made from three separate directions. This enables three components ofwind speed to be determined from three different directions therebyproviding a 3-D wind field measurement of probe volume 124 to beascertained. Such 3-D wind field information can be advantageous in manysituations; for example ship-landing applications. In ship-landingapplications, the CLR device could also be located on the ship; the windfield information being communicated to the aeroplane or helicopter asrequired.

FIG. 5 b illustrates a device of the type described with reference toFIG. 3 deployed to make wind speed measurements on a wind farm thatcomprises multiple wind turbines 130.

The central unit 64 of the CLR may be positioned within one of the windturbines (e.g. wind turbine 130 c in FIG. 4 b), and the transceivers 84are linked to the central unit 64 via optical fibre cables 82. Eachtransceiver 84 is mounted on each wind turbine 130, and detects the windspeed in probe volumes 132 that are a certain distance in front of eachwind turbine. This allows the wind turbines to react to the windstrength they are about to encounter, thereby preventing turbine damageand optimising power generation efficiency.

Although the device described with reference to FIG. 3 is shown in thevarious applications described with respect to FIG. 5, this should in noway be seen as limiting. Any device based on those described withreference to FIGS. 2, 3 or 4 could be used for the applicationsdescribed with reference to FIG. 5. Similarly, a person skilled in theart would recognise CLR devices of the present invention could beapplied to situations other than those described with reference to FIG.5.

1. A wind velocity measurement device comprising a laser source, and twoor more transceivers; and at least one detector, wherein radiationreceived by said two or more transceivers is coherently mixed withradiation extracted from the laser source prior to detection by the atleast one detector, characterised in that the two or more transceiversare optically connected to the laser source by optical fibre cables, theoptical fibre cables connecting the laser source and the two or moretransceivers are routed through a first optical routing means; andwherein the wind velocity measurement device is arranged to measure thewind speed in front of two or more wind turbines wherein one of said twoor more transceivers is located in or each of said two or more windturbines.
 2. A device as claimed in claim 1 wherein the first opticalrouting means is a multi-way optical splitter.
 3. A device as claimed inclaim 1 wherein the first optical routing means is an optical switch. 4.A device as claimed in claim 3 wherein the optical switch is awavelength division multiplexer (WDM), the WDM being arranged so as todirect laser radiation of different wavelengths to differenttransceivers.
 5. A device as claimed in claim 3 wherein the opticalswitch is a fibre switch that is controlled to direct laser radiation toa particular transceiver head.
 6. A device as claimed in claim 1 whereina single detector is provided, and the radiation received by the two ormore transceivers is optically coupled to the detector by optical fibrecables via a second optical routing means.
 7. A device as claimed inclaim 6 wherein the second optical routing means is an optical switch.8. A device as claimed in claim 7 wherein the optical switch is a WDM,and the frequency of the laser is alterable such that radiation receivedby a particular transceiver head is directed to the detector.
 9. Adevice as claimed in claim 7 wherein the optical switch is a fibreswitch that is controlled such that radiation received by a particulartransceiver head is directed to the detector.
 10. A device as claimed inclaim 1 wherein radiation received by the two or more transceivers isdirected to two or more detectors.
 11. A device as claimed in claim 9wherein the detectors are coupled to each transceiver by an opticalfibre cable.
 12. A device as claimed in claim 1 wherein the transceiversare monostatic.
 13. A device as claimed in claim 1 wherein thetransceivers are bistatic.
 14. A device as claimed in claim 1 havingmore than three transceivers.
 15. A device as claimed in claim 1 havingmore than five transceivers.
 16. A device as claimed in claim 1 havingmore than ten transceivers.
 17. A device as claimed in claim 1 whereinthe transceivers are located remotely to the laser source.
 18. A devicea claimed in claim 1 wherein the laser source is a semiconductor laser.19. A device as claimed in claim 1 wherein the laser source is asemiconductor laser and an erbium doped amplifier arranged in a matteroscillator power amplifier configuration.
 20. A device as claimed inclaim 1 wherein the laser source outputs radiation with a wavelengthbetween 1.4 μm and 1.6 μm.
 21. A device as claimed in claim 1 whereinthe laser source is frequency modulated and the detected signals areprocessed to provide range information.
 22. A device as claimed in claim1 wherein the detected signals are processed to provide vibrometricinformation.
 23. A device as claimed in claim 1 wherein the apparatusused to process the detected signals is located remotely to thetransceivers.
 24. A device as claimed in claim 1 wherein the apparatusused to process the detected signals is located substantially adjacentto the laser source.
 25. A wind speed measurement system for use on anaircraft incorporating a device as claimed in claim 1 wherein thetransceivers are located along the wings of the aircraft and aredirected so as to measure the wind speed of probe volumes in front ofthe aircraft, the measured wind speed information being used to adjustthe configuration of the aircraft wings.
 26. A wind speed measurementsystem for use in aircraft landing applications incorporating a deviceas claimed in claim 1 that has at least three transceivers, wherein thetransceivers are spatially separated and are adapted to measure the windspeed of a single probe volume, the measurement of the wind speed of aprobe volume from three different directions permitting the threedimensional wind field vector for the probe volume to be ascertained.27. A wind velocity measurement for measuring wind strength in front oftwo or more wind turbines incorporating a device comprising: a lasersource; two or more transceivers; and at least one detector, whereinradiation received by said two or more transceivers is coherently mixedwith radiation extracted from the laser source prior to detection by theat least one detector, characterised in that the two or moretransceivers are optically connected to the laser source by opticalfibre cables, the optical fibre cables connecting the laser source andthe two or more transceivers are routed through a first optical routingmeans wherein the two or more transceivers are located in each windturbine and the laser source is located in the base of one of the windturbines and wherein the detected signals are processed to obtain speedinformation and wherein the speed of particles in air is determined toprovide wind speed measurements.
 28. A device according to claim 1wherein the optical fibre cable connecting the two or more transceiversand the laser source is several kilometers in length.
 29. A deviceaccording to claim 28 wherein at least one optical isolator is locatedalong the optical fibre.
 30. A coherent laser radar device comprising: alaser source; two or more transceivers; and at least one detectorwherein radiation received by said two or more transceivers iscoherently mixed with radiation extracted from the laser source prior todetection by at least one detector, wherein the two or more transceiversare optically connected to the laser source by optical fibre cables, andwherein the optical fibre cables connecting the laser source and the twoor more transceivers are routed through a wavelength divisionmultiplexer (WDM), the WDM being arranged so as to direct laserradiation of different wavelength to different transceivers.